1. Field of the Invention
The present invention relates to an image data area extracting system and an image data area extracting method applicable to image diagnosis devices. Particularly, the present invention is directed to an image data area extracting system and an image data area extracting method for removing areas that are unnecessary for diagnosis in a medical image.
2. Discussion of the Background
Generally, an image diagnosis apparatus, such as an X-ray computed tomography (CT) device, is used to acquire and reconstruct X-ray CT image data by detecting X-rays penetrated through an object. In particular, to observe blood vessels, imaging is performed by injecting a contrast medium into an object and X-ray CT image data of the contrasted blood vessels is then reconstructed. Namely, three-dimensional (3D) projection image data is constructed using the reconstructed X-ray CT image data.
Usually, a Maximum Intensity Projection (MIP) method, a minimum intensity projection method (MinIP), or an X-ray projection method is used for reconstructing the 3D projection image data. To observe the contrasted blood vessels, 3D projection images are generally constructed using the MIP method. The resultant image is known as an MIP image.
While an MIP image is useful to observe blood vessels, it has several limitations. More particularly, images of other physical structures, such as bones, are reconstructed with the blood vessel data because a CT value for a contrasted blood vessel and a CT value for a bone are close. As a result of the presence of the bone tissue images in the MIP images, accurate diagnosis of blood vessels is inhibited.
To resolve this difficulty, another method has been proposed, for example, as described in Japanese Patent Application Publication No. 9-73557. According to this method, 3D images of surface rendering (SR) images using voxel or volume rendering (VR) images are reconstructed. Additionally, data related to unnecessary bone areas is deleted through the SR images or the VR images prior to observation of the contrasted blood vessels.
FIGS. 1(a)-1(c) illustrate the method for deleting unnecessary bone area prior to observation of the contrasted blood vessels. FIG. 1(a) illustrates a shaded volume rendering (SVR) image produced using the MIP method. Because the CT value of bone is generally equal to the CT value of a blood vessel in the conventional method, both blood vessels and bones appear in the SVR image as shown in FIG. 1(a).
To discriminate between bone and blood vessels in an SVR image, a threshold value using CT value and opacity for a CT value are set. The image is displayed by discriminating between bone and contrasted blood vessels using the threshold and opacity. It is also possible to delete bone area image data selectively by dividing or by extracting SVR image data of bone area using a connected area extracting method.
FIG. 1(b) shows an SVR image of a contrasted blood vessel from which bone area image data has been deleted. The MIP image is produced from X-ray CT image data included in the blood vessel area of the SVR image.
However, these observations of contrasted blood vessels using an X-ray CT apparatus cannot efficiently perform the deletion of bone area image data because many soft tissues present in the same area have CT values substantially equal to CT values for bones. For example, other tissues (e.g., cartilage or other organs) may interfere with the removal of bone area image data.
Because the CT value of bone ranges to a certain extent, if a threshold value and opacity are set at a low CT value at the start of bone area image data deletion operation, organs (which are a type of soft tissue) are also displayed with bones. Since soft tissues (e.g., internal organs) include large contact areas with the contrasted blood vessels, it is difficult to extract the bone area image data. Thus, if CT values of the threshold value and the opacity are set low, it is difficult to efficiently delete the bone area image data.
An alternative approach is to set the CT value of the threshold and opacity high. While the bone area image data is deleted by setting the threshold value at high CT to construct the MIP images from the X-ray CT image data, as depicted in the circled ranges of 1(c), soft tissue (e.g., cartilage) remains in the images, since the CT value of cartilage is substantially similar to the CT value of bones.
It is possible construct an MIP image by selectively deleting cartilage based on setting a new threshold value with the connected area extracting method. By doing this, it is possible to delete the bone area image data that remains in the MIP image in FIG. 1(c). However, even in this reconstructed MIP image, cartilage having lower CT values remains. Consequently, to obtain an MIP image effective for diagnosis, it is necessary to repeat complex processes to delete the bone area image data using the 3D image, a confirmation of the MIP images, and successive setting of threshold values for deletion.
While the above examples have illustrated the difficulties for diagnosing blood vessels, the same difficulties also apply to observation of other organs. Additionally, the data used for processing in the above-identified application may be obtained from a local memory, a remote memory, or a storage device via a communication network, such as in a PACS system.